Fill in the grid so that every row, every column, and every 3 x 3 box contains the digits 1 through 9. Additional the rows form 9 correct arithmetic expressions reading from left to right.
There is only one solution

Fill the grid with the digits 1 to 9. The digits represent the height of the skyscraper in each cell. Each row, column and 3x3-box will have exactly one of each digit. The clues along the edges tell you how many skyscrapers you can see from that vantage point.

Upper centre - Min Max Quad Sudoku: The red points in the near of crosses where four cells meet each other show that the cell with the red points is greater then the three other ones. The blue rings in the near of crosses where four cells meet each other show that the cell with the blue ring is smaller then the three other ones.

Upper right - Arrow Sudoku: The digit in the circle is the sum of the digits which are marked with its arrow.

Centre left - Difference Sudoku: The numbers between two cells give the difference of the digits.

Centre - Navigator Sudoku: The arrows show the location of the same number in the previous or next column or row. So for example, the arrow in fifth column means that the same digit is in the sixth column in row 6 - 9.

Centre right - Greater/Less Sudoku: The inequality between each pair of digits must stand correct.

Fill the grid with the digits 1 to 9. The digits represent the height of the skyscraper in each cell. Each row, column and 3x3-box has exactly one of each digit. The clues along the edges tell you how many skyscrapers you can see from that vantage point. The digit in a circle is the sum of the digits which are marked with its arrow.